The fiber chromatic dispersion is a result of dependence of the fiber refractive index on the signal wavelength. Since the optical wave velocity in a fiber is given by
      V    ⁡          (      λ      )        =      c          n      ⁡              (        λ        )            
where V(λ) is the optical signal velocity, c is the light speed in vacuum and n(λ) is the fiber refractive index, the signal velocity also depends on the signal wavelength.
Due to the final spectral width of any optical pulse signal, different parts of the signal will propagate through the fiber with different velocities causing pulse distortion, which will be called the signal dispersion in the frame of the present application. As a result of this, various effects appear. For example, such effects are caused by mutual interference between adjacent optical pulses within the optical channel (so-called inter-symbol interference ISI), and decrease and spread the pulse peak. These effects are considered harmful, and specific techniques are usually required for overcoming them.
For compensating the fiber dispersion, one may use fibers with the dispersion characteristics opposite to those of the standard fiber. Such fibers are usually called dispersion compensating fibers (DCF).
One alternative technique for compensating the fiber dispersion uses chirped periodic structures to create different delays between signals of different wavelengths and therefore to compensate for the fiber chromatic dispersion. This technique is presented today by the chirped fiber Bragg gratings, for example described in a Japanese patent application JP 20002 35170 A. Arrangements belonging to this technique do not create non-linear interactions, the gratings have a small size and allow for creation of variable compensation modules.
U.S. Pat. No. 6,252,692 describes an optical transmission system including means for measuring optical dispersion in an optical path, and a controllable element such as a dispersion compensator, operable in dependence on the measured value of dispersion. A low frequency dither on the optical signal causes timing jitter which varies according to the dispersion in the optical path. The timing jitter is extracted from a clock signal recovered from the optical signal. This jitter is correlated with the original dither to remove jitter effects caused by other mechanisms. Thus a value for dispersion is derived which can be used for monitoring or control purposes. The dispersion determining means comprises means for impressing a pattern on the transmitted signal; the system also comprises correlating means to correlate the timing jitter with this pattern. Provided that the pattern is asymmetric, the correlating means are arranged to determine the sign of the dispersion value.